There are various types of media used for the storage of data. Each media type has particular characteristics that typically dictate the environment/application that it is best suited for. For example, disk media is typically used for real-time data storage when fast access to a particular location on the media is required. Tape media, and in particular magnetic tape, is typically used for off-line data storage of large amounts of data such as a backup or archive copy of data. Disk media is relatively expensive when compared to other types of media such as tape. Tape has the disadvantage of a relatively slow access time, when compared to disk, as the tape is wound about a reel and must be accessed serially by either forwarding or rewinding the tape to the desired location for reading/writing data. It would be desirable to improve the access time for tape media such that the cost benefit of tape could be used in more types of environments/applications that traditionally use disk (with its associated faster access time and lower latency). In addition, with appropriate management of the data, the technique for improving serially accessible media's access time can provide the additional benefit of data redundancy.
As seen in FIGS. 1A–1C, tape media used for the storage of computer data can be packaged in many different forms. FIG. 1A shows a single reel tape cartridge, such as that used with a 9490 tape drive offered by Storage Technology Corporation, headquartered in Louisville, Colo. This tape cartridge 12 has a single supply reel 13 and hub 14 contained therein, with a portion 15 of the tape media 16 wrapped around hub 14 when in a loaded position (i.e. threaded and ready for access by a transducer 20 such as a magnetic read/write head). Another portion of the tape 16 is shown threaded along a tape path defined by a plurality of rollers or capstans 18. Another portion 17 of the tape is shown as being wound about a hub 24 within take-up reel 22. This tape is shown to be in a loaded position within a tape drive—where the tape is adjacent to the transducer 20 for data access by the tape drive. Motors (not shown) are used to drive the hubs 14 and 24 such that the tape can be positioned in a forward or reverse direction such that different linear portions of the tape can be positioned adjacent the transducer 20 for data access (e.g. reading or writing). When in an unloaded position, for example when the cartridge is not loaded in a tape drive, the tape would not extend along the tape path, but rather be exclusively contained within the tape cartridge 12. In a single reel cartridge system, the take-up reel 22 is contained within and is a part of the tape drive.
FIG. 1B shows a dual reel tape cartridge 112, such as that used with a 9840 tape drive also offered by Storage Technology Corporation. This tape cartridge 112 has both a supply reel 113/hub 114 and a take-up reel 122/hub 124 contained therein. Tape 116 is shown threaded along a tape path defined by a plurality of rollers or capstans 118. When loaded in a tape drive, the tape is accessible by a transducer (not shown) via aperture 100. Motors (not shown) are used to drive the hubs 114 and 124 such that the tape can be positioned in a forward or reverse direction such that different linear portions of the tape can be positioned adjacent a transducer for data access (e.g. reading or writing).
FIG. 1C shows a dual reel cassette 212, such as that originally used with a VCR tape recorder/player traditionally used for viewing/recording programming on/off a television or monitor, and now being used for the storage of data using helical recording. Such helical data storage is described in U.S. Pat. No. 5,128,815, which is hereby incorporated by reference as background material. There is a supply reel 213/hub 214 and take-up reel 222/hub 224, with one end of tape 216 attached to the supply reel 213, and the other end of tape 216 attached to the take-up reel 222. Again, a plurality of rollers or capstans 218 are used to define a tape path, which in this instance positions tape outside of cartridge 212 to wrap the tape about a helical transducer 220.
U.S. Pat. No. 6,061,194 describes a technique for writing duplicate data at a fixed azmith angle from the original data on a disk platter, in order to reduce rotational latency when reading the data. This duplicate data is written on the same platter as the original data, and the media is relatively expensive when compared to tape.
It would be advantage to provide a technique for improving access time for serially accessible storage media, and to improve data redundancy in a storage system having such media. Examples of serially accessible media include magnetic tape, optical tape, and charge coupled device (CCD) shift registers.